The first generation Grubbs catalyst, while largely replaced by the second generation catalyst in usage, was not only the first catalyst to be developed other than those developed by Richard R. Schrock (Schrock carbenes), but is also important as a precursor to all other Grubbs-type catalysts.

The second generation catalyst has the same uses in organic synthesis as the first generation catalyst, but generally with higher activity. This catalyst is stable toward moisture and air, thus is easier to handle in the lab.

Shortly before the discovery of the 2nd generation Grubbs' catalyst, a very similar catalyst based on an unsaturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)imidazole) was reported independently by Nolan[9] and Grubbs[10] in March 1999, and by Fürstner[11] in June of the same year. Shortly thereafter, in August 1999, Grubbs reported the 2nd generation catalyst, based on a saturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)dihydroimidazole):[12]

In both the saturated and unsaturated cases a phosphine ligand is replaced with an N-heterocyclic carbene (NHC), which is characteristic of all 2nd generation type catalysts.[3]

Both the 1st and 2nd generation catalysts are commercially available, along with many derivatives of the 2nd generation catalyst.

In the Hoveyda–Grubbs Catalysts, the benzylidene ligands have a chelating ortho-isopropoxy group attached to the benzene rings. The ortho-isopropoxybenzylidene moiety is sometimes referred to as a Hoveyda chelate. The chelating oxygen atom replaces a phosphine ligand, which in the case of the 2nd generation catalyst, gives a completely phosphine-free structure. The 1st generation Hoveyda–Grubbs catalyst was reported in 1999 by the Hoveyda group,[13] and in the following year, the 2nd generation Hoveyda–Grubbs catalyst was described in nearly simultaneous publications by the Blechert[14] and Hoveyda[15] laboratories. Blechert's name is not commonly included in the eponymous catalyst name. The Hoveyda–Grubbs catalysts, while more expensive and slower to initiate than the Grubbs catalyst from which they are derived, are popular because of their improved stability.[3] Hoveyda–Grubbs catalysts are easily formed from the corresponding Grubbs catalyst by the addition of the chelating ligand and the use of a phosphine scavenger like copper(I) chloride:[15]

The 2nd generation Hoveyda–Grubbs catalysts can also be prepared from the 1st generation Hoveyda–Grubbs catalyst by the addition of the NHC:[14]

The initiation rate of the Grubbs' catalyst can be altered by replacing the phosphine ligand with more labile pyridine ligands. By using 3-bromopyridine the initiation rate is increased more than a million fold:[17]

The principle application of the fast-initiating catalysts is as initiators for ring opening metathesis polymerisation (ROMP). Because of their usefulness in ROMP these catalysts are sometimes referred to as the 3rd generation Grubbs' catalysts.[18] The high ratio of the rate of initiation to the rate of propagation makes these catalysts useful in living polymerization, yielding polymers with low polydispersity.[19]

Olefin metathesis is a reaction between two molecules containing double bonds. The groups bonded to the carbon atoms of the double bond are exchanged between molecules, to produce two new molecules containing double bonds with swapped groups. Whether a cis isomer or trans isomer is formed in this type of reaction is determined by the orientation the molecules assume when they coordinate to the catalyst, as well as the sterics of the substituents on the double bond of the newly forming molecule.